1. Field of the Invention
The present invention relates generally to adhesives used to bond electronic components to circuit boards, and, more particularly, to flexible epoxy-based compounds having high electrical resistivity and improved thermal transfer characteristics.
2. Description of Related Art
The design and fabrication of electronics often involve bonding the electronic components to circuit boards using flexible epoxy adhesives. U.S. Pat. No. 4,866,108, assigned to the present assignee, discloses and claims one such flexible epoxy adhesive composition. The composition comprises a mixture of a fatty acid modified epoxy resin and an oxypropylene polyether epoxy resin in the ratio of about 1:3 to 1:1. The adhesive also includes a stoichiometric amount of a polyamine curing agent, 1 to 20 total liquid weight percent of a plasticizer, 50 to 80 weight percent of aluminum oxide, and 1 to 5 weight percent of microfine silicon dioxide particles. This adhesive is particularly well-suited for use as a flat pack adhesive to provide a releasable bond and is also well-suited for use in space applications. While this adhesive is very useful for its intended purposes, it has volume resistivity values which are typically 10.sup.10 to 10.sup.12 ohm-cm at 25.degree. C. It has been found that certain electronic circuits, such as multiplexers, require volume resistivities greater than 10.sup.13 ohm-cm at 25.degree. C. and greater than 10.sup.11 ohm-cm at 95.degree. C. to minimize cross-talk and other problems.
Further, this prior adhesive evidences glass transition temperatures on the order of -10.degree. to -20.degree. C. Such glass transition temperatures indicate the possibility of the filleting material becoming rigid in its service temperature range, which could result in the cracking of the bonded components, such as glass-bodied diodes. Lower glass transition temperatures, on the other hand, imply that the filleting material remains elastomeric throughout the service temperature range. Lower glass transition temperatures are needed to retain flexibility over the application temperature range of about -50.degree. to 125.degree. C. By "lower" glass transition temperatures is meant less than about -30.degree. C.
Accordingly, a need remains for a flexible film adhesive having a volume resistivity in excess of 10.sup.13 ohm-cm at 25.degree. C. and in excess of 10.sup.11 ohm-cm at 95.degree. C., low temperature flexibility, excellent adhesion, and ease of reworkability.